As described in our patent application GB2482110, a common approach to boost the conductivity of the anode connection of a large-area organic light-emitting diode (OLED) lighting tile is to use a matrix of electrically conductive tracks. As shown in FIG. 1a, a lighting tile 200 generally has (anode) electrode tracks defining a hexagonal or square grid 102, with an electrical busbar 104 surrounding the grid (the opening shown is to facilitate connection with the counter electrode).
FIG. 1b illustrates that within a given area of the tile, the current primarily flows through the anode electrode in a single dominant direction. For example, if the current is supplied from all edges of the tile, the predominant direction of the current is towards the laterally central position of the lighting tile. As the highest voltage drop appears at the edges of the lighting tile where electrical busbars are connected to the tracks, providing more metal at the edges reduces the voltage drop. However this also generally results in a decrease in aperture ratio. Here the aperture ratio may be defined as: (1−the area of the electrodes)/(the total active device area).
In GB2482110, we addressed this problem by providing tracks which subdivide into a plurality of tracks with increasing distance from a busbar, thus increasing the conductivity towards the electrical busbars where greater conductivity is needed. An example of this approach is shown in FIG. 1c. Furthermore, it is desirable to minimise the voltage drop across the tile when being in operation, in particular in large-area devices. As the light-emitting area of the tile increases, the level of voltage drop for a constant electrical resistance becomes more apparent. This voltage drop manifests itself in a luminance drop across the panel.
There remains, however, a general need for improving the luminance profile of a lighting tile while maximising the aperture ratio.